I'm wondering, if it possible to use the scattering boundary conditions in the RF module to make a boundary transparent for a plane wave with a known wave-vector (which is different from the wave vector of the incident field at this boundary).

To give a simple example, if I, for example, want to simulate the reflections of a plane wave (with a wave vector given by k=(kx,0,kz) and a polarisation of E=(1,0,-kx/kz)) by a metallic half space (surface given by z=0), I know that the wave vector of the reflected field will be k=(kx,0,-kz). Is there any possible to make this simple simulation using scattering boundary conditions (without using PML) and solving for the full field? (I don't want to solve for the scattered field or use PML, since it would be inconvenient for the simulations I'm interested in. Obviously my final goal is not to calculate the reflection of a plan wave at a metal surface.)

Thanks a lot for your quick and friendly reply. Unfortunately, I'm still struggling with the problem. Probably I should explain in more detail what I try to do. This is also the reason while I changed the subject of the thread slightly.

I set up a model adapted to the problem I'm actually interested in by using the weak-form interface of Comsol. Now I want to use one boundary of the problem to excite a plane wave within the simulation area (I make 2d simulations), but this boundary should be also transparent for the reflected wave. Using the notation of my first post the weak contribution is given by the cross product of H and n (the normal vector). Assuming that the magnetic(H) and electric(Ex,Ez) field at the boundary are given by an unknown reflected (Hr,Erx,Erz) and a know incident (Hi,Eix,Eiz) contribution, one ends up with

H=1/ (mu0*omega)*(-kz*Ex-kx*Ez+2*kz*Eix)

But using this weak contribution I only get reasonable results for kx=0, in the other cases the reflected wave is reflected again at the boundary of the simulation area resulting in an interference pattern (see the attached image).

I'm not sure how to do it using the weak form. Can you avoid using it? Then port BC would work just fine. If you still want to have weak-form, then check the equations in port BC and try to do something similar.